Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of gaseous and solid compounds, including particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. The amount of pollutants emitted from an engine may be regulated depending on the type, size, and/or class of engine.
One method that has been implemented by engine manufacturers to comply with the regulation of NOx exhausted to the environment has been to implement a strategy called selective catalytic reduction (SCR). SCR is a process by which gaseous or liquid reductant (e.g., a mixture of urea and water) is injected into the flow of exhaust from an engine. The combined flow may form ammonia which may then be absorbed onto a catalyst. The ammonia may react with NOx in the flow of exhaust to form H2O and N2, thereby reducing the amount of NOx in the flow of exhaust.
The ability of urea to form ammonia for reducing NOx at the catalyst depends on the temperature in the exhaust system. At high temperatures (e.g., greater than approximately 200° C.), urea may form ammonia. At low temperatures (e.g., less than approximately 160° C.), urea may be deposited on the pipe walls and may be unable to form ammonia. At moderate temperatures (e.g., approximately 160° C. to 200° C.) and after an extended period of time, urea may be unable to form ammonia and may instead form decomposition materials, such as cyanuric acid, biuret, malamine, and/or other polymeric precursors. These polymeric precursors may react after extended periods of time to form highly stable polymeric materials that may only be removed by physical means, such as a chisel or grinder.
One method of preventing the accumulation of the urea in the exhaust pipe and/or on the catalyst face is described in U.S. Patent Publication No. 2007/0044457 (the '457 publication) to Upadhyay et al. The '457 publication describes an exhaust gas aftertreatment system that includes a catalyst including a housing, SCR catalyst bricks, and an electric heater embedded into the housing. The heater may be activated for a predetermined amount of time to desorb ammonia from the heated region of the catalyst. The temperature of the catalyst may be calculated based on temperature measurements upstream and downstream of the catalyst. To prevent accumulation of the urea in the exhaust pipe and/or on the catalyst face, reductant injection into the catalyst stops when the calculated catalyst temperature is less than a predetermined temperature threshold (170° C.).
Although the system of the '457 publication may prevent the injection of urea when the calculated catalyst temperature is below the predetermined temperature threshold, when the catalyst temperature falls under the predetermined temperature threshold, urea that has already been deposited on the walls of the catalyst may still form the polymeric precursors. These polymeric precursors may form polymeric materials that may only be removed by chisel or grinder, thereby requiring potentially time-consuming and costly maintenance on the catalyst, requiring periodic replacement of the catalyst, and/or decreasing the effectiveness of the catalyst in reducing the amount of NOx in the flow of exhaust.
The disclosed system is directed to overcoming one or more of the problems set forth above.